U.S. patent number 7,829,892 [Application Number 11/926,653] was granted by the patent office on 2010-11-09 for integrated circuit including a gate electrode.
This patent grant is currently assigned to Qimonda AG. Invention is credited to Franz Hofmann, Richard Johannes Luyken, Dirk Manger, Lothar Risch, Wolfgang Roesner, Till Schloesser, Michael Specht.
United States Patent |
7,829,892 |
Luyken , et al. |
November 9, 2010 |
Integrated circuit including a gate electrode
Abstract
An integrated circuit including a gate electrode is disclosed.
One embodiment provides a transistor including a first source/drain
electrode and a second source/drain electrode. A channel is
arranged between the first and the second source/drain electrode in
a semiconductor substrate. A gate electrode is arranged adjacent
the channel layer and is electrically insulated from the channel
layer. A semiconductor substrate electrode is provided on a rear
side. The gate electrode encloses the channel layer at least two
opposite sides.
Inventors: |
Luyken; Richard Johannes
(Munich, DE), Hofmann; Franz (Munich, DE),
Risch; Lothar (Neubiberg, DE), Manger; Dirk
(Dresden, DE), Roesner; Wolfgang (Ottobrunn,
DE), Schloesser; Till (Dresden, DE),
Specht; Michael (Munich, DE) |
Assignee: |
Qimonda AG (Munich,
DE)
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Family
ID: |
33394203 |
Appl.
No.: |
11/926,653 |
Filed: |
October 29, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080054324 A1 |
Mar 6, 2008 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10839800 |
May 6, 2004 |
7368752 |
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Foreign Application Priority Data
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May 7, 2003 [DE] |
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103 20 239 |
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Current U.S.
Class: |
257/71; 257/343;
257/E21.646; 257/E27.084 |
Current CPC
Class: |
H01L
27/10826 (20130101); H01L 27/10823 (20130101); H01L
27/10891 (20130101); H01L 27/10808 (20130101); H01L
27/10829 (20130101); H01L 29/7851 (20130101) |
Current International
Class: |
H01L
29/12 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Chenming Hu, "SOI and nanoscale MOSFETs", Device Research
Conference, Jun. 25-27, 2001, pp. 3-4. cited by other .
Tai-su Park et al., "A 40 nm body-tied FinFED (OMEGA MOSFET) using
bulk Si wafer", Physica E 19 (2003) pp. 6-12,
www.elsevier.com/locate/physe. cited by other.
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Primary Examiner: Stark; Jarrett J
Attorney, Agent or Firm: Dicke, Billig & Czaja, PLLC
Claims
What is claimed is:
1. An integrated circuit, comprising: a transistor, comprising a
first source/drain electrode; a second source/drain electrode; a
channel arranged in a longitudinal direction between the first and
the second source/drain electrode in a semiconductor substrate, the
channel disposed vertically within the semiconductor substrate in a
direction from an upper surface to a rear surface of the
semiconductor substrate; a gate electrode, which is arranged
adjacent the channel layer and is electrically insulated from the
channel layer; and a semiconductor substrate electrode arranged on
a rear side of the semiconductor substrate, wherein the gate
electrode is disposed vertically within the semiconductor substrate
and includes a first gate electrode disposed on a first lateral
side of the channel and a second gate electrode disposed on a
second lateral side of the channel such that the gate electrode
encloses the channel on least the two opposite lateral sides.
2. The integrated circuit of claim 1, wherein the gate electrode
includes a horizontal portion disposed on a top side of the channel
between the channel and the upper surface of the semiconductor
substrate such that the gate electrode is formed approximately in
U-shaped fashion in cross section and encloses the channel at three
sides.
3. The integrated circuit of claim 1, wherein the top side of the
channel is adjacent to and space from the horizontal upper surface
of the semiconductor substrate.
4. The integrated circuit of claim 1, wherein the channel is formed
essentially as a web on the semiconductor substrate.
5. The integrated circuit of claim 1, wherein the channel has an
essentially homogeneous doping over the channel height.
6. The integrated circuit of claim 1, wherein the channel doping
has a doping atom concentration of at most 1*10.sup.17 cm.sup.-3
and a doping atom concentration of at least 5*10.sup.17 cm.sup.-3
is present below the channel.
7. The integrated circuit of claim 6, wherein the channel length in
the longitudinal direction corresponds at least to 2.5 times the
channel width in the lateral direction.
8. The integrated circuit of claim 1, wherein the channel doping
decreases over the channel length toward the source/drain electrode
connected to a capacitor electrode, the doping atom concentration
at the source/drain electrode connected to the capacitor electrode
being at most 5*10.sup.17 cm.sup.-3.
9. The integrated circuit of claim 8, wherein the channel length in
the longitudinal direction corresponds at least to the channel
width in the lateral direction.
10. The integrated circuit of claim 1, wherein a top side of the
channel extends along and is spaced from a planar surface of the
semiconductor substrate.
11. The integrated circuit of claim 1, wherein a top side of the
channel is spaced from and extends approximately horizontally with
respect to the semiconductor substrate.
Description
CLAIMS FOR PRIORITY
This application claims the benefit of priority to German
Application No. 103 20 239.0, filed in the German language on May
7, 2003, and of U.S. patent application Ser. No. 10/839,800, filed
May 7, 2003, both of which are incorporated herein by
reference.
TECHNICAL FIELD OF THE INVENTION
The invention relates to a DRAM memory cell having a planar
selection transistor and a storage capacitor connected to the
planar selection transistor.
BACKGROUND OF THE INVENTION
In order to obtain a sufficiently large read signal of the DRAM
memory cell, the storage capacitor has to provide a sufficient
storage capacitance. On account of the limited memory cell area,
storage capacitors which utilize the third dimension are therefore
used. One embodiment of such a three-dimensional storage capacitor
is a so-called trench capacitor, which is arranged in a manner
laterally adjoining the selection transistor, preferably
essentially below the selection transistor, the inner capacitor
electrode arranged in a trench being electrically conductively
connected to the selection transistor. A further embodiment of a
three-dimensional storage capacitor is the so-called stacked
capacitor, which is likewise arranged in a manner laterally
adjoining the selection transistor, preferably essentially above
the selection transistor, the inner capacitor electrode being
conductively connected to the selection transistor.
The selection transistor in the DRAM memory cell is generally a
junction transistor in which two highly conductive doping regions
are diffused into the semiconductor substrate and serve as
current-supplying (source) and current-receiving (drain)
electrodes, a current-conducting channel between source and drain
electrodes being formed between the two doped regions with the aid
of a gate electrode isolated by an insulating layer, in order to
write and read the charge to and from the storage capacitor.
As the areas of the memory cells become smaller and smaller on
account of increasing miniaturization, retaining the current driver
capability of the transistor poses an increasing problem. Current
driver capability of the transistor is understood to be the
transistor's property of supplying, in the case of a predetermined
source/drain potential and a predetermined gate voltage, a
sufficient current in order to charge the storage capacitor
sufficiently rapidly. However, the shrinking of the cell areas and
the resultant shrinking of the transistor dimensions mean that the
transistor width of the planar junction transistors decreases. This
in turn has the effect of reducing the current switched through
from the transistor to the storage capacitor. One possibility of
retaining the current driver capability of the planar transistor
with a reduced transistor width consists in correspondingly scaling
the gate oxide thickness or the doping profile of the source/drain
regions and of the channel region. However, there is the problem of
increased leakage currents when the gate oxide thickness is reduced
or the doping concentrations are higher.
As an alternative to planar DRAM selection transistors, therefore,
vertically arranged transistors are increasingly being discussed in
order, in the case of selection transistors, too, additionally to
be able to utilize the third dimension and obtain larger transistor
widths. In the case of such a vertical selection transistor, which,
in the case of an assigned trench capacitor, is arranged
essentially directly over the trench capacitor and, in the case of
an assigned stacked capacitor, is arranged essentially directly
under the stacked capacitor, there is, in particular, the
possibility of enclosing the channel region of the transistor
almost completely with the gate electrode, as a result of which the
current driver capability per transistor area can be significantly
increased. However, vertically embodied transistors are very
complicated in terms of process engineering and can be fabricated
only with difficulty, in particular with regard to the connection
technique of the source/drain regions and of the gate electrodes of
the transistor. What is more, there is the problem that, during the
operations of switching the selection transistor on and off, the
semiconductor substrate is also concomitantly charged at the same
time, and the so-called floating body effect occurs, as a result of
which the switching speed of the transistor is greatly impaired. In
order to prevent this, the semiconductor substrate is generally
provided with a substrate connection in order to ensure that the
semiconductor substrate is discharged during the transistor
switching operations. In the case of vertical selection
transistors, however, there is the problem that even with the aid
of such substrate connections, the semiconductor substrate can
often be discharged only to an inadequate extent.
Furthermore, in particular in connection with logic circuits, new
junction transistor concepts are known which can achieve a higher
current intensity relative to the transistor width in comparison
with the conventionally planar transistors. One possible
short-channel junction transistor concept is the so-called double
gate transistor, in which the channel region between source and
drain regions is encompassed by a gate electrode at least on two
sides, whereby a high current driver capability can be achieved
even in the case of very short channel lengths since an increased
channel width results in comparison with conventional planar
selection transistors. In this case, it is preferred for the double
gate transistor to be designed as a so-called Fin-FET, in which the
channel region is embodied in the form of a fin between the source
and drain regions, the channel region being encompassed by the gate
electrode at least at the two opposite sides. Given a suitable
design of the fin width and thus of the channel width, such a
Fin-FET can be operated in such a way that, in the turned-on state
with an applied gate electrode voltage, the two inversion layers
that form under the gate electrodes overlap and a complete charge
carrier inversion thus takes place, as a result of which the entire
channel width can be utilized for current transport. What is more,
Fin-FETs afford the possibility of directly controlling the
so-called short-channel effects, which occur in the case of very
short channel lengths and may lead to an alteration of the
threshold voltage of the transistor, by means of the gate potential
instead of, as in the case of conventional planar FETs, through the
need to provide special doping profiles in the channel region of
the transistor. An improved control of the short-channel effects is
thus achieved with the aid of the Fin-FET. Furthermore, Fin-FETs
are distinguished by a large subthreshold gradient and thus a good
switch-on and switch-off behavior in conjunction with a reduced
subthreshold leakage current. Not having to control short-channel
effects by means of the channel doping additionally makes it
possible to reduce the channel doping and thus to achieve a high
channel mobility and a high threshold voltage.
Double gate transistors, in particular Fin-FETs, are generally
fabricated on an SOI substrate (SOI=silicon on insulator) in order
to avoid impairing the electrical properties of the double gate
transistors. In the case of an SOI substrate, the silicon layer in
which the transistor is formed is isolated from the underlying
semiconductor wafer by a buried insulator layer. This configuration
has the disadvantage that when the double gate transistor is
intended to be used as a selection transistor for a DRAM cell, the
silicon layer is charged as a result of the transistor being
switched on and off, which significantly impairs the switching
speed of the transistor. Although it is possible to avoid such
charging of the silicon layer with the Fin-FET by means of an
additional electrical connection, said additional connection can be
effected only directly via the silicon surface, which results in an
increased area requirement on account of the additional connection
area, which is at odds with the desired miniaturization of the DRAM
memory cell.
SUMMARY OF THE INVENTION
The invention relates to a DRAM memory cell having a planar
selection transistor and a storage capacitor connected to the
planar selection transistor. The stored information is represented
by the charge of the storage capacitor, the storage states 0 and 1
corresponding to the positively and negatively charged storage
capacitor, respectively. The storage capacitor is written to and
read from by switching on the selection transistor. Since the
capacitor charge of the storage capacitor decreases very rapidly on
account of recombination and leakage currents, the charge is
generally refreshed again with millisecond timing.
The present invention provides a DRAM memory cell with a reduced
area requirement, the selection transistor formed in planar fashion
being distinguished by a high current driver capability and
charging of the semiconductor substrate being avoided at the same
time.
According to another embodiment of the invention, a DRAM memory
cell is formed having a selection transistor, which is arranged
horizontally at a semiconductor surface and has a first
source/drain electrode, a second source/drain electrode, a channel
layer arranged between the first and the second source/drain
electrode in the semiconductor substrate, and a gate electrode,
which is arranged along the channel layer and is electrically
insulated from the channel layer, the gate electrode enclosing the
channel layer at at least two opposite sides. The selection
transistor configured in this way is connected to a storage
capacitor, which has a first capacitor electrode and a second
capacitor electrode, insulated from the first capacitor electrode,
one of the capacitor electrodes of the storage capacitor being
electrically coupled to one of the source/drain electrodes of the
selection transistor, and a further substrate electrode being
provided on the rear side.
In the design according to the invention, in which a double gate
transistor is formed directly on the semiconductor substrate
without the interposition of an insulator layer, affords the
possibility of using such a double gate transistor, which is
distinguished by a high current driver capability, relative to the
channel length, and improved electrical properties, particularly in
the case of a short channel, in the case of DRAM memory cells and
at the same time of providing for the possibility of using a
semiconductor substrate electrode on the rear side to avoid
charging of the semiconductor substrate as a result of the
switching operations of the selection transistor.
In accordance with one preferred embodiment of the invention, the
gate electrode is formed essentially in U-shaped fashion in cross
section and encompasses the channel layer at three sides, as a
result of which it is possible to achieve a higher current through
the selection transistor and at the same time an improved control
of short-channel effects. In this case, it is preferred for the
gate electrode to be electrically conductively connected to a word
line running transversely over the channel layer, as a result of
which a particularly compact construction of the Fin-FET selection
transistor is achieved.
In accordance with a further preferred embodiment, the channel
layer is formed essentially in web-type fashion, the channel doping
being embodied essentially homogeneously over the channel layer
height. This ensures a threshold voltage of the selection
transistor that is independent of the height of the channel.
In accordance with a further preferred embodiment, a doping of the
channel web over the semiconductor substrate is embodied in such a
way that the channel layer doping has a doping concentration of at
most 1.times.10.sup.17 cm.sup.-3 over the height of the gate
electrodes, while a doping concentration of at least
5.times.10.sup.17 cm.sup.-3 is embodied below the channel layer
toward the semiconductor substrate. Such a doping profile enables a
full depletion mode of the selection transistor, a high carrier
mobility and thus a good current flow being ensured by the low
doping in the channel region. At the same time, the high doping
below the channel region toward the semiconductor substrate ensures
that, in the case of high drain/source voltages, no breakdown
occurs between the source and drain regions below the channel since
the increased doping in this region provides for a sufficient
blocking effect. In the case of such a channel doping with an
elevated buried doping layer below the channel layer, it is
possible to form double gate transistors with a channel layer
length which corresponds to 2.5 times the channel layer
thickness.
In accordance with a further preferred embodiment, the channel
layer doping in the direction toward the source/drain electrode
connected to the capacitor electrode is designed such that the
doping atom concentration decreases, the doping atom concentration
in the region of said source/drain electrode being at most
5.times.10.sup.17 cm.sup.-3. This design makes it possible to
produce particularly short channel lengths since a relatively
strong pn junction is present at the source/drain electrode
connected to the bit line and provides for a rapid field decrease
of the source/drain voltage, the low doping at the electrode
connected to the capacitor electrode simultaneously ensuring that a
sufficient charge carrier current can flow into the capacitor
electrode. A channel doping configured in this way makes it
possible to achieve channel layer lengths which have to correspond
to the channel layer width.
In accordance with a further preferred embodiment, the storage
capacitor of the DRAM memory cell is formed three-dimensionally
either as a trench capacitor, which is arranged essentially below
the Fin-FET selection transistor, or as a stacked capacitor, which
is arranged essentially above the Fin-FET. The use of such
three-dimensional storage capacitors provides for a sufficient
storage capacitance in conjunction with a minimal area requirement
for the memory cell.
It is furthermore preferred, in the case of a DRAM memory cell
array, to arrange the DRAM memory cells in matrix-type fashion on
the semiconductor substrate, in which case, when using trench
capacitors, the trench capacitors are preferably arranged regularly
in rows and the trench capacitors of adjacent rows are offset with
respect to one another. After the formation of the trench
capacitors, which are preferably provided with a buried plate,
double gate selection transistors assigned to the trench capacitors
are then formed such that firstly a strip-type hard mask layer is
produced parallel to the rows of trench capacitors, the hard mask
layer strips being arranged essentially between the rows of trench
capacitors and the trench capacitors being partly covered.
Afterward, spacer layers are produced at the steps of the hard mask
layer strips and the uncovered semiconductor surfaces are etched
down to a predetermined depth by means of anisotropic etching in
the region between the hard mask layer strips and the adjoining
spacer layers. The etched-free regions are then in turn filled with
spacer layer material, the hard mask layer strips are subsequently
removed and the surfaces uncovered under the hard mask layer strips
are opened down to the predetermined depth by means of anisotropic
etching. Afterward, the spacer layer material is then completely
removed and an insulator layer is produced in large-area fashion.
After the application of a polysilicon layer and the embodiment of
a gate electrode patterning, the source/drain dopings are produced.
By virtue of this procedure, DRAM memory cells having trench
capacitors and double gate selection transistors can be produced in
a simple manner using conventional DRAM process steps.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in more detail with reference to the
accompanying drawings, in which;
FIG. 1 shows a circuit diagram of a dynamic memory cell.
FIG. 2 shows a dynamic memory cell according to the invention with
Fin-FET and trench capacitor.
FIG. 2A shows a cross section of the embodiment in FIG. 2.
FIG. 2B shows a longitudinal section of the embodiment in FIG.
2.
FIG. 3 shows a DRAM memory cell according to the invention with a
Fin-FET and a stacked capacitor.
FIG. 3A shows a cross section of the embodiment in FIG. 3.
FIG. 3B shows a longitudinal section of the embodiment in FIG.
3.
FIG. 4 shows configurations according to the invention of Fin-FETs
as DRAM selection transistor.
FIG. 4A shows a diagrammatic cross section through a Fin-FET.
FIG. 4B shows input characteristic curves on a logarithmic scale
for various Fin-FET designs.
FIG. 5 shows a first fabrication process according to the invention
for forming a DRAM memory with Fin-FETs as selection transistors
and trench capacitors as storage capacitors.
FIGS. 5A to 5E illustrate cross sections through the semiconductor
wafer after different process steps.
FIG. 6 shows a second fabrication process according to the
invention for forming a DRAM memory with Fin-FETs as selection
transistors and trench capacitors as storage capacitors.
FIGS. 6A to 6D illustrate a plan view and a cross section through
the semiconductor wafer after successive process steps.
DETAILED DESCRIPTION OF THE INVENTION
Dynamic memory cells are composed of a selection transistor and a
storage capacitor. The storage states 0 and 1 correspond to the
positively and negatively charged capacitor, respectively. However,
the capacitor charge in the DRAM memory cells decreases after a few
milliseconds on account of recombination and leakage currents, so
that the charge of the capacitor has to be repeatedly refreshed.
After a read operation, too, the information has to be regularly
rewritten to the capacitor of the DRAM memory cell.
FIG. 1 shows the circuit diagram of a DRAM memory cell having a
storage capacitor 1 and a selection transistor 2. In this case, the
selection transistor 2 is preferably formed as a normally off
n-channel field-effect transistor (FET) and has a first n-doped
source/drain electrode 21 and a second n-doped source/drain
electrode 23, between which an active weakly p-conducting region 22
is arranged. A gate insulator layer 24 is provided over the active
region 22, a gate electrode 25 being arranged over the gate
insulator layer, which gate electrode acts like a plate capacitor
and can be used to influence the charge density in the active
region 22.
The second source/drain electrode 23 of the selection transistor 2
is connected to the first electrode 11 of the storage capacitor 1
via a connecting line 4. A second electrode 12 of the storage
capacitor 1 is in turn connected to a capacitor plate 5, which is
preferably common to the storage capacitors of a DRAM memory cell
arrangement. The first electrode 21 of the selection transistor 2
is further connected to a bit line 6 in order that the information
stored in the storage capacitor 1 in the form of charges can be
read in and out. A read-in and read-out operation is controlled via
a word line 7 connected to the gate electrode 25 of the selection
transistor 2 in order, by application of a voltage, to produce a
current-conducting channel in the active region 22 between the
first source/drain electrode 21 and the second source/drain
electrode 23. In order to prevent the semiconductor substrate from
being charged during the operations of switching the transistor on
and off, a substrate connection line is further provided.
In the case of dynamic memory cells, the storage capacitors used
are in many cases three-dimensional structures, in particular
trench capacitors, which are arranged essentially below the
selection transistor, and stacked capacitors, which are arranged
essentially over the selection transistor, it thereby being
possible to achieve a significant shrinking of the memory cell
area. Even with a minimal memory cell area, such three-dimensional
storage capacitors ensure a sufficiently large storage capacitance
of approximately 25 to 40 fF, which provides for reliable detection
of the information stored in the storage capacitor.
One difficulty in the case of the progressive shrinking of the cell
area results, however, from the need to ensure a sufficient current
driver capability of the selection transistor in order that the
storage capacitors can be charged sufficiently rapidly. Selection
transistors in DRAM memory cells are generally formed as planar
n-channel field-effect transistors, two highly conductive n-type
regions being diffused into a p-conducting semiconductor substrate
and serving as current-supplying source electrode and
current-receiving drain electrode. A dielectric layer, preferably a
silicon dioxide layer, is applied over the region between the two
highly n-conducting regions, the preferably metallic gate electrode
being provided over said layer. Progressive miniaturization of such
planar field-effect transistors gives rise to the problem that the
current intensity, relative to the ever shorter channel lengths, no
longer suffices to provide for rapid charging of the storage
capacitors. What is more, there is the problem that a possible
improvement of the current driver capability of planar transistors
by reducing the gate oxide thickness or increasing the doping
profiles would lead to intensified leakage currents.
According to the invention, therefore, the planar selection
transistor is formed as a so-called double gate field-effect
transistor, as a result of which it is possible to achieve
significantly higher current intensities relative to the channel
length in comparison with the conventional planar transistors.
FIGS. 2 and 3 show two possible designs of a double gate
field-effect transistor in a DRAM memory cell.
FIG. 2 illustrates a DRAM memory cell construction with a trench
capacitor 100 as storage capacitor. The trench capacitor 100 has an
inner capacitor electrode 101, which is preferably formed as a
n-doped polysilicon filling. The inner capacitor electrode 101 is
isolated from an outer capacitor electrode 103 by a dielectric
layer 102, the outer capacitor electrode preferably being formed as
a buried n-type doping in a semiconductor substrate 10 surrounding
the trench capacitor. The upper region of the trench capacitor is
surrounded by a thick insulation layer, preferably an oxide collar
104, which prevents an electrical short circuit between the buried
outer capacitor electrode 103 and a selection transistor that
controls the trench capacitor. The trench capacitor 100 is
furthermore covered by an insulating covering layer 105.
The selection transistor 200, which is formed as a double gate
field-effect transistor and is designed as a normally off
n-MOS-FET, is arranged beside the trench capacitor 100 in the
weakly p-doped semiconductor substrate 10. As shown in FIG. 2B, in
particular, the selection transistor 200 has two highly n-doped
regions 201, 202 at the semiconductor surface, which lie
essentially in one plane with the trench capacitor. The two highly
n-doped regions 201, 202 serve as first and second source/drain
electrodes, the second source/drain electrode 202 being connected
to the inner capacitor electrode 101 via a conductive connection
106 in the insulation collar 104, preferably a heavily n-doped
polysilicon region. A channel region 203 is provided between the
first and the second source/drain electrode 201, 202, which channel
region is embodied in the form of a web in the semiconductor
substrate 10, as shown by the cross section in FIG. 2A. Said
channel region 203 extends between the first and the second
source/drain electrode 201, 202 far into the semiconductor
substrate 10 and, in a lower region 204, is laterally surrounded by
a thick insulator layer 205, preferably an oxide layer, which is
adjoined laterally by a thin gate oxide 206 in the upper channel
region 203. The thin gate oxide 206 separates the upper channel
region 203 from two lateral gate electrode sections 207 which
encompass the upper channel region and are in turn laterally
adjoined by a word line layer 70. In this case, the word line 70
runs essentially transversely with respect to the DRAM memory cell.
An insulator layer 208, preferably a silicon nitride layer, is
provided as a covering layer on the selection transistor 200, in
which layer, in turn, a bit line 60 is arranged essentially along
the DRAM memory cell, the bit line being connected to the first
source/drain electrode 201 via a conductive contact connection 61.
A substrate connection 90 is furthermore provided at the rear side
of the semiconductor substrate 10.
FIG. 3 shows a second embodiment of a DRAM memory cell according to
the invention with a double gate transistor. In this embodiment, as
shown in particular by the longitudinal section in FIG. 3B, the
storage capacitor 300 is formed as a stacked capacitor arranged
essentially over a selection transistor 400. In this case, the
stacked capacitor 300 has an inner capacitor electrode 301 at the
semiconductor surface 10, which electrode has, in cross section,
essentially the form of a crown (only partly shown) and preferably
comprises a highly n-doped polysilicon layer. The inner capacitor
electrode 301 is enclosed by a dielectric layer 302, which is in
turn bordered by an outer capacitor electrode 303 (only partly
shown) preferably embodied in block-type fashion, which outer
capacitor electrode is formed as a highly n-doped polysilicon
layer. The inner capacitor electrode 301 is connected via a contact
block 304, preferably a highly n-doped polysilicon layer, to a
second source/drain electrode 402 of the selection transistor 400
formed as a double gate FET.
The Fin-FET 400 is formed essentially along the semiconductor
surface below the stacked capacitor 300 with two highly n-doped
regions in the semiconductor substrate 10, which serve as first
source/drain electrode 401 and as second source/drain electrode
402. An essentially plate-type channel region 403 is provided
between the two highly doped regions 401, 402 and, as shown by the
cross section in FIG. 3A, is formed as a web on the semiconductor
substrate 10. In its lower region 404, the channel region is
laterally bordered by an insulator layer 405, preferably an oxide
layer, which is adjoined by a thin gate oxide layer 406
peripherally around the upper region of the channel 403. Said gate
oxide layer 406 isolates the gate electrode 407, which is likewise
formed around the channel region on three sides and is connected to
a word line layer 71, which is formed over the gate electrode and
runs essentially transversely with respect to the DRAM memory
cell.
An insulator layer 408, preferably a silicon nitride layer, is in
turn provided on the word line 71. The first source/drain electrode
401 of the double gate selection transistor is connected via a
conductive contact block 63, preferably a highly doped polysilicon
layer to a bit line 62, which runs essentially transversely with
respect to the DRAM memory cell and is separated from the outer
capacitor electrode 303 of the stacked capacitor 300 by a further
insulator layer 64, preferably an oxide layer. An electrode region
91 for connection of the semiconductor substrate 10 is provided on
the rear side of the semiconductor substrate.
The solution according to the invention of a DRAM memory cell
having a storage capacitor that is preferably formed
three-dimensionally and a selection transistor formed as a double
gate field-effect transistor, the channel region of which is formed
in the semiconductor substrate, the semiconductor substrate in turn
being provided with a substrate connection, makes it possible, even
in the case of short channel lengths, to ensure a sufficient
current intensity between the source and drain regions of the
double gate transistor and at the same time to prevent charging of
the semiconductor substrate during the switching operations. The
DRAM memory cell according to the invention can be restricted to a
small substrate surface, a sufficient current driver capability
with which the capacitor can be charged sufficiently rapidly
simultaneously being ensured. Forming the double gate transistor
directly on the semiconductor substrate as a web, the semiconductor
substrate being provided with a substrate connection, ensures that
the so-called floating body effect, i.e. charging of the
surrounding semiconductor substrate, does not occur when the
selection transistor is switched on and off.
The double gate transistor according to the invention can be
fabricated simply and cost-effectively in the context of the known
DRAM fabrication processes through simple modification of the
process sequence for forming planar selection transistors. The
selection transistor according to the invention, formed as a double
gate field-effect transistor, is furthermore distinguished by
improved electrical properties in comparison with conventional
planar field-effect transistors. The gate electrode sections
arranged on both sides of the channel afford the possibility of
utilizing the entire channel width for forming a conductive channel
layer for turning on the selection transistor, since charge carrier
inversion can take place in the channel over the entire channel
width and the entire channel can thus be utilized for current
conduction. At the same time, such a so-called full depletion mode
results in a good switch-on and switch-off behavior on account of
the resultant high subthreshold gradient in conjunction with a low
subthreshold leakage current. What is more, the short-channel
effects that occur in the case of the short channel lengths can be
controlled in a simple manner through the voltage control of the
two lateral gate regions without having to provide a high doping in
the channel region. This in turn ensures that a high threshold
voltage and at the same time a high charge carrier mobility and
thus a fast switching behavior of the selection transistor are
achieved.
By means of suitable doping profiles of the channel region of the
double gate field-effect transistor according to the invention, it
is furthermore possible to improve the current driver capability
and also its switching behavior. FIG. 4A shows a cross section
through a transistor structure which essentially corresponds to the
first embodiment shown in FIG. 2 with a web-like channel region 500
on the semiconductor substrate, which is laterally enclosed in a
lower region 504 by an insulator layer 502 adjoined by a thin gate
oxide layer 503, which separate lateral gate electrode sections 507
from an upper channel region 501. In this case, the channel region
has a channel width W and a channel height Z, corresponding to the
height of the gate electrode section 507.
FIG. 4B shows, on a logarithmic scale, input characteristic curves
on such a Fin-FET in the case of a channel length L of 50 nm and a
channel width W of 20 nm. In this case, the source/drain electrodes
are arsenic-doped n-type regions having a doping concentration of
2.times.10.sup.20 cm.sup.-3. The silicon substrate 10 with the
channel region lying between the source/drain electrodes is weakly
p-doped, preferably with boron with a doping concentration of
5.times.10.sup.13 cm.sup.-3, the doping decreasing from the first
source/drain electrode, connected to the bit line, toward the
second source/drain electrode, connected to the storage capacitor,
preferably with a gradient of 3.5 nm/dec. Furthermore, the doping
increases under the channel toward the substrate with a rise of 14
nm/dec. The channel height is 200 nm.
FIG. 4B illustrates the source/drain current I.sub.d for two
source/drain voltages U.sub.d 0.1 and 1 volt and for three
different depths of the source/drain implantation of 50 nm, 100 nm
and 200 nm relative to the gate voltage U.sub.g. It is found in
this case that a shallow doping, in comparison with a deep doping
of the source/drain regions, leads to a lower current flow but to
an improved breakdown behavior and vice versa. Therefore, the
doping depth of the source/drain regions is preferably chosen in
such a way as to ensure a current intensity that is high enough for
charging the capacitor whilst at the same time avoiding a breakdown
between source/drain electrode in the selection transistor.
Furthermore, FIG. 4B reveals that the design according to the
invention with a double gate field-effect transistor leads to a
good subthreshold gradient of approximately 75 mv/dec.
In one preferred embodiment, the double gate field-effect
transistor according to the invention is formed such that the
channel layer has an essentially homogeneous doping with a doping
concentration of 1.times.10.sup.17 cm.sup.-3, a doping
concentration of 5.times.10.sup.17 cm.sup.-3 being present in the
web region below the gate electrodes. Such a doping profile makes
it possible to achieve a
channel-layer-length-to-channel-layer-width ratio of 2.5, a
sufficiently high current intensity simultaneously being ensured
whilst avoiding a breakdown below the channel region.
In accordance with a second preferred embodiment, a doping profile
which decreases toward the source/drain electrode connected to the
capacitor electrode is provided in the channel layer, the doping
concentration in the region of the source/drain electrode connected
to the capacitor electrode being at most 5.times.10.sup.17
cm.sup.-3. Such a doping gradient of the channel layer makes it
possible to achieve a channel-layer-length-to-width ratio of 1, a
sufficiently high current intensity for charging the capacitor
simultaneously being ensured whilst preventing a breakdown below
the channel layer.
FIGS. 5A to E show a possible process sequence for forming a
dynamic memory cell according to the invention in a DRAM memory,
the memory cell being provided with a trench capacitor. In this
case, the individual structures of the dynamic memory cell are
preferably formed with the aid of silicon planar technology, which
comprises a sequence of individual processes acting in each case
over the whole area at the surface of a silicon semiconductor
wafer, a local alteration of the silicon substrate being carried
out in a targeted manner by means of suitable masking layers. A
multiplicity of dynamic memory cells are preferably formed
simultaneously during the DRAM memory fabrication. The invention is
explained below using the example of forming two memory cells that
are connected to one another via a common bit line. FIGS. 5A to 5E
in each case show a cross section through the silicon wafer after
the last individual process respectively described. In this case,
the process steps for forming the dynamic memory cell which are
essential to the invention are discussed below. Unless explained
otherwise, the structures are formed in the context of the
customary DRAM process sequence.
FIG. 5A shows a cross section through the silicon wafer, which is
preferably a monocrystalline silicon substrate 10 having a weak
p-type doping. Trench capacitors 100, corresponding to the trench
capacitors shown in FIG. 2A, are embodied in the silicon wafer 10.
The trench capacitors are fabricated in the context of conventional
trench processing by means of photolithography technology, a
one-sided trench connection 106 in each case being formed at
opposite sides. In this case, the two trench capacitors 100 shown
are embodied in such a way that the trenches are filled with a
highly n-doped polysilicon layer, preferably arsenic or phosphorus
being used for doping, the filling serving as an inner capacitor
electrode 101. In the lower region, the polysilicon filling 101 is
surrounded by a dielectric layer 102, which may comprise a stack of
dielectric layers and is distinguished by a high dielectric
constant.
A highly n-doped layer 103, serving as a second capacitor
electrode, is formed in turn around the dielectric layer 102. A
collar oxide layer 104 is formed around the inner capacitor
electrode 101 in a manner adjoining the dielectric layer 102, the
capacitor connection 106 being provided in said collar oxide layer
on one side. The trench capacitor 100 is furthermore covered with
an oxide layer 105. A substrate connection 90, preferably in the
form of a highly p-doped region, is formed on the rear side of the
weakly p-doped semiconductor substrate 10. A thin oxide layer 109
is additionally provided around the trench capacitors on the
semiconductor surface.
In a further process sequence, selection transistors designed as
double gate field-effect transistors are then formed between the
two trench capacitors 100. For this purpose, after eliminating the
oxide layer 109, by means of a first lithography step, the channel
layer formed in web-type fashion is defined in the silicon
substrate 10. Afterward, trenches are embodied in the semiconductor
substrate by means of an anisotropic etching, which trenches define
the channel layer regions. The etching depth is depicted in dotted
fashion in FIG. 5B. After eliminating the photolithography mask, a
thin oxide layer 110 is in turn formed on the silicon wafer 10. A
cross section through the silicon wafer after this process step is
shown in FIG. 5B.
In a further process sequence, a gate oxide layer is then produced
by oxidation laterally around the etched-free channel layers and a
polysilicon deposition is subsequently performed in order to
produce the gate electrodes. Furthermore, a high n-type doping,
preferably with phosphorus, is embodied in the polysilicon layer.
After a gate lithography in which the regions of the gate
electrodes are defined around the channel layer but spaced apart
from the two trench capacitors, the gate electrodes 207 with the
underlying gate oxides are etched free. Over the gate electrodes
207, the word lines are then fabricated, in a manner running
transversely with respect to the memory cells, in the form of a
further highly doped polysilicon layer 170. FIG. 5 shows a cross
section through the semiconductor wafer in which four word lines
170 are formed on the semiconductor surface, two over the
corresponding gate electrodes 207 of the double gate field-effect
transistors and two over the laterally arranged trench capacitors
100, which serve for the connection of the next row of DRAM memory
cells arranged in the form of a checkerboard. The word lines 170
are enclosed by a silicon spacer layer 171 formed by application of
a silicon nitride layer and subsequent etching-back. A cross
section through the silicon wafer after the spacer processing is
shown in FIG. 5C.
Through the remaining silicon oxide layer 110, the source/drain
electrodes 201, 202 of the n-channel transistors are then embodied
e.g. by means of ion implantation with arsenic. A cross section
through the silicon wafer with the highly n-doped source/drain
electrodes is shown in FIG. 5D. In this case, three doped regions
are formed between the two trench capacitors 100, the two doping
regions 202 adjoining the trench capacitors serving as second
source/drain electrodes of the two selection transistors 200. The
highly n-doped region 201 formed between the two channel regions
serves as a common first source/drain electrode for both selection
transistors 200. The common source/drain electrode 201 is then
connected to a bit line in a further process sequence, an oxide
layer 111 being applied in a first process step, a metal block 161
for making contact with the first source/drain electrode 201 being
embodied in said oxide layer in a self-aligning manner, the bit
line track 160 being embodied, in turn, on said metal block in a
manner such that it runs transversely. A cross section through the
silicon wafer after this process step is shown in FIG. 5E.
An alternative embodiment for fabricating a DRAM memory cell
according to the invention in a DRAM memory having a double gate
field-effect transistor and a trench capacitor is illustrated in
the process sequence 6A to 6D. The individual figures show in each
case a diagrammatic plan view of the silicon wafer 10 and a cross
section after the last process step respectively explained. In this
case, in a manner similar to the process sequence illustrated in
FIG. 5, an arrangement of trench capacitors 100 is embodied on the
silicon wafer 10, a multiplicity of trench capacitors being
arranged regularly in rows and adjacent rows of trench capacitors
being embodied in offset fashion. Each trench capacitor 100 has an
inner capacitor electrode 101, which is preferably embodied as a
highly n-doped polysilicon block separated from an outer electrode
103, embodied as a doping region in the lower region (not shown),
by a lateral dielectric layer 102. A block-type oxide covering
layer 105 is embodied on the trench capacitor 100, the layer being
surrounded by a silicon nitride layer 112. The silicon wafer with
the trench capacitors 100 embodied in this way is illustrated in
FIG. 6A.
In a next process step, a hard mask lithography process is then
used to fabricate strip-type hard mask layers 113, preferably made
of SiON or a so-called low-K material, the hard mask layers 113
running in strip-type fashion parallel to the rows of trench
capacitors 100. In this case, the hard mask layer strips 113 are
arranged essentially between the rows of trench capacitors and
partly cover the trench capacitors. Spacer layers 114 are produced
at the steps of the hard mask layer strips 113 by application of an
oxide layer and subsequent etching-back. A plan view of the
semiconductor wafer and a detail cross section after this process
step are illustrated in FIG. 6B.
An anisotropic etching step is performed next in order to open the
surface that is uncovered between the hard mask layer strips 113
and the adjoining spacer layers 114 as far as a predetermined depth
in the silicon substrate 10. In a further process step, the
etched-free region between the hard mask layer strips 113 and the
adjoining spacer layers 114 is then in turn filled with the silicon
dioxide used as spacer layer material and the hard mask layer
strips are then removed. Trenches having the same depth as in the
first etching step are then once again embodied by means of
subsequent anisotropic etching of the surface that is uncovered
under the hard mask layer strips. The spacer layer material is then
removed. A plan view of the semiconductor wafer and a cross section
through the semiconductor wafer after this process step are shown
in FIG. 6C.
In a further process sequence, silicon dioxide 115 is then applied
in large-area fashion as insulator layer and gate oxide layer. A
polysilicon layer 116 is subsequently deposited and planarized. The
polysilicon layer 116 is doped and patterned in a further
lithography process in order to form the lateral gate electrodes
and the word lines running transversely, in a manner similar to
that in the case of the process sequence illustrated in FIG. 5. In
the uncovered regions between the word lines with the underlying
gate electrodes, the source/drain implants are then embodied and
subsequently covered with an insulator layer 117, through which one
source/drain electrode of the transistor is then contact-connected
to a subsequently applied bit line 260 with the aid of contact
blocks. A plan view and a cross section through the silicon wafer
after this concluding process step for forming the dynamic memory
cells are shown in FIG. 6D.
In addition to the two process sequences shown with reference to
FIGS. 5 and 6 for forming dynamic memory cells with a
three-dimensional storage capacitor and a planar double gate
selection transistor, it is also possible to have recourse to other
process sequences for forming three-dimensional storage capacitors
and double gate selection transistors. Furthermore, it is possible
for the conductivity type of the doped regions in the memory cells
to be embodied in complementary fashion. What is more, the
materials specified for forming the various layers may be replaced
by other materials that are known in this connection.
* * * * *
References